There exist a variety of membranes that are capable of separating a component from a feed gas stream when such membranes function at temperatures in excess of 250° F. Such membranes can function to transport the component to be separated from one side of the membrane, known as the retentate side, to the other side thereof, referred to as the permeate side. For example, in hydrogen separation, membranes, utilized to separate hydrogen, are formed of a thin layer of palladium or alloy of palladium on a porous supporting material. At elevated temperatures, when a feed stream containing hydrogen contacts the retentate side of the membrane, hydrogen atoms will diffuse through the palladium lattice to the opposite, permeate side, and emerge as pure hydrogen.
An example of a hydrogen transport membrane can be found in U.S. Pat. No. 5,652,020, which describes a hydrogen transport membrane comprised of a palladium layer deposited on a porous ceramic support layer. In addition to the foregoing, certain ceramic materials are capable of functioning as hydrogen transport membranes by conducting protons under the impetus of a partial pressure difference. Examples of such membranes can be found in U.S. Pat. Nos. 6,066,307 and 6,037,514. Porous membranes can also be used to selectively transport hydrogen based on molecular characteristics, such as size and shape. Examples of such membranes can be found in U.S. Pat. Nos. 6,527,833 and 7,074,734.
It is to be noted that in the use of hydrogen transport membranes, the hydrogen transport membrane can be combined with a process that is designed to produce the hydrogen containing stream from which the hydrogen is separated. For example, in U.S. Pat. No. 6,783,750, a reactor is disclosed in which oxygen produced by oxygen transport membranes is reacted with a hydrocarbon containing feed and steam to produce a synthesis gas from which the hydrogen is separated from the synthesis gas with the use of a hydrogen transport membrane. The permeate side of the hydrogen transport membrane is swept with steam to lower the partial pressure of the hydrogen on the permeate side and help drive the hydrogen separation across the membrane.
As indicated above, the separation of the component with the use of such membranes is driven by a partial pressure difference of the component on opposite sides of the membrane. This partial pressure difference can be established by compressing the feed stream containing the component and/or by introducing a sweep gas stream to the permeate side of the membrane to remove the separated component such as shown in U.S. Pat. No. 6,783,750. The use of a sweep gas stream has the advantage of decreasing the compression requirement for the feed gas stream and therefore the electrical power consumed in the separation process. Since it is only the partial pressure difference that is needed to drive the separation, the sweep gas stream can be introduced at pressure to allow the component to be delivered at such pressure after separation from the sweep gas material. Additionally, for a given separation, the membrane area can be reduced when a sweep gas stream is used. This is particularly advantageous with respect to palladium membranes given the expense of palladium.
While the sweep gas could be a compressed gas, it is more advantageous to use a liquid that has been pressurized by pumping and then vaporizing the liquid into a gas. One advantage is that, typically, pumps have much lower capital and operating costs compared to compressors. Moreover, when the component is to be separated from the component laden sweep gas stream, such stream can be condensed so that the component may be removed as a resulting vapor phase of the component laden sweep gas. The problem with this is that heat must be supplied to vaporize the pumped liquid that cannot be easily recovered. For example, steam has been used as a sweep gas stream in connection with palladium hydrogen transport membranes. However, once the hydrogen or other component that is separated has been added to the steam, the partial pressure of the steam drops, lowering its condensation temperature. Given that the vaporization and condensation temperature of steam at a given pressure is the same, for example, 100° C. at a partial pressure equal to atmospheric pressure, and that the vaporization and condensation temperature is a direct function of partial pressure of water vapor, transferring the latent heat of vaporization between the component laden steam and the makeup water is not possible. Steam will condense at a lower temperature than that required to vaporize the water. Therefore, when steam is used as the sweep stream, the sweep stream must be superheated to such an extent that there will be a large temperature difference in the heat exchanger. The problem is that the energy expended in heating the sweep stream in the case of steam is particularly high to maintain a temperature difference within a heat exchanger and further, the heat cannot be easily recovered. This is because although the degree of superheating of the steam may be sufficient to vaporize water, once a component is added to the steam, the condensation temperature will decrease and most of the steam will not condense in the heat exchanger. The subsequent condensation of the steam will result in what is in effect lost heat that is not recovered. Alternatively, if the sweep stream does not contain sufficient superheat to boil the water, a large portion of the steam might be condensed in the heat exchanger to heat the water to near its boiling point but additional high temperature heat will be required to boil the water. Significant energy losses will result from producing the high temperature heat needed to boil the water.
As will be discussed the present invention provides a method and apparatus in which a gas-impermeable membrane is swept using a sweep stream that is designed to allow heat energy to be recovered in a more efficient manner than the prior art discussed above.